import numpy as np
A=np.arange(3,15).reshape((3,4))
print(A)
print(A[2][1])
print(A[2,1])
print(A[2,:])
print(A[:,1])
print(A[1,1:3])
print(A[2]) 
for row in A:
    print(row)
for column in A.T: #迭代列要先转置
    print(column)  
print(A.flatten()) #返回一个一行的数组   
for item in A.flat:#返回一个迭代器
    print(item)      